I think their animated characters are expensive. All cutscenes with a lot of characters cheat by using many static character models.

Also the game has to run on ps4, which is a pretty limited platform by today's standards. And when you make a ps4 game, it is a mandatory requirement from sony that you should never drop below 30 fps. Obviously that probably happens in the open world when a lots of players are around, but it's very possible that Sony could have agreed to waive that requirement in those situations, but not during "solo" gameplay.